Link Layer Protocol and Error Correction

Data Link Layer (Layer 2 of OSI)

• The data link layer is responsible for transferring datagrams from one node to an adjacent node over a link.
• The layer's services include framing, link access, reliable delivery, flow control, and error detection.

Error Detection and Correction

• Error detection involves adding error checking bits (redundancy) to data to detect errors.
• Error correction involves identifying and correcting bit errors without resorting to retransmission.
• Error detection is not 100% reliable, and protocols may miss some errors.

Internet Checksum

• The internet checksum is used to detect errors in transmitted segments at the transport layer.
• The sender computes the checksum by adding the segment contents as 16-bit integers and storing the value in the UDP checksum field.
• The receiver checks if the computed checksum equals the checksum field value; if not, an error is detected.

Link Layer Services

• Framing involves encapsulating datagrams into frames, adding headers and trailers.
• Link access controls channel access if shared medium.
• 'Physical addresses' (MAC addresses) are used in frame headers to identify source and destination nodes.

Reliable Delivery

• Reliable delivery between adjacent nodes involves ensuring that data is delivered correctly and in the correct order.

Flow Control

• Flow control involves pacing between adjacent sending and receiving nodes to prevent data overflow.

LAN Technologies

• LAN technologies include Ethernet, hubs, bridges, switches, 802.11, PPP, and ATM.
• The data link layer is responsible for addressing in LANs.

LAN Addresses and ARP

• LAN addresses (MAC addresses) are used to get datagrams from one interface to another physically-connected interface.
• 32-bit IP addresses are used to get datagrams to destination IP networks.
• LAN addresses are 48-bit addresses burned in the adapter ROM.
• MAC addresses are allocated by the IEEE to ensure uniqueness.